1. Field of the Invention
The invention relates generally to an actuator arm apparatus in a disc drive system and to a method of installing the outer actuator arms at a particular optimum distance from the memory disc surface of their corresponding memory discs to compensate for the differences in gram loads on the head gimbal assembly and improve the performance of the servo motor system.
2. Description of Prior Art
Disc drive systems read and write information stored on concentric circular tracks on memory discs. Information or data is stored on the surface of the memory discs via a read/write media. The data is divided or grouped together in tracks. Transducers, in the form of read/write heads, located on both sides of the memory disc, read and write information on the memory discs when they are accurately positioned over one of the designated or target tracks on the surface of the memory disc. As the memory disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing onto the memory disc in a particular manner. Similarly, reading data on a memory disc is accomplished by positioning the read/write head above a target track and reading the stored material on the memory disc. To write on or read from different tracks, the read/write head is moved radially across the concentric tracks to the target track.
Typical disc drives have a plurality of memory discs spaced apart and rotating about a common spindle. Because of the importance of positioning the read/write head substantially parallel to the memory disc surface, a head gimbal assembly is installed on an actuator arm. An actuator arm comprises a flexure, a head gimbal assembly, and a mounting support. The read/write head is found at the distal end of the actuator arm. The proximal end of the actuator arm is coupled to a pivot assembly. The pivot assembly is in turn connected to a servo motor system. The flexure and head gimbal assembly allow the read/write head to gimbal for pitch and roll to follow fluctuations in the imperfect memory disc surface but restrict motion in the radial and circumferential directions of the memory disc. The flexure is coupled to an mounting support coupled to a servo motor. As the disc drive system sends control signals to the motor, the motor rotates, thereby displacing the actuator arm supporting the read/write head across the memory disc in a radial direction to the target track. The control signals indicate to the motor the magnitude and direction of the displacement.
Disc drive systems are very high precision units requiring close dimensional tolerances in manufacturing. In recent years, the size of disc drives have decreased from a 14-inch form factor to 1.8-inch form factor. In contrast, the density of information (bits per inch and tracks per inch) stored on memory discs compatible with such disc drives has increased. To meet the increased density requirements, the recording performance of the read/write head must be optimized. The performance of the read/write head is a function of the distance between the head and the disc surface where the data is stored. Currently, this spacing must be typically 3.0 microinches.
Fly-height refers to this spacing as the disc rotates and the read/write head "flies" across the disc surface to position itself over the target track. To ensure proper performance, the head must maintain this fly-height of 3.0 microinches. Fly height is determined by gram load, pitch static attitude, and the appropriate placement of the actuator arm over the disc surface (z-height).
Gram load refers to the load on the head gimbal assembly. When the disc is rotating, no gram load is applied to the head gimbal assembly. When the disc has ceased rotating, a portion of the head gimbal assembly makes contact with the disc surface and as a result, gram load is applied to the head gimbal assembly. The z-height of the mounting support affects the gram load; the lower the z-height, the greater the gram load.
Pitch static attitude refers to the ability of the read/write head to "ride" with the fluctuations of the laminar flow of air generated by disc rotation and with the uneven disc surface. More particularly, pitch refers to the vertical angular displacement of the head about the horizontal plane of its "flight path." The read/write head is mounted on a "slider" which has an air bearing surface positioned immediately adjacent the flat surface of the memory disc. As the memory disc spins, the laminar flow of air following the memory disc lifts the slider and the read/write head by applying vertical air pressure onto the air bearing surface of the slider. Ideally, any displacements in distance by the slider are negligible compared to the distance of the actuator arm from the disc surface.
Actuator arms act as spring-mass-damper systems and have resonant frequencies that can degrade the performance of the servo system. Every closed loop servo motor system has a predetermined bandwidth in which resonances occurring within the bandwidth degrade the performance of the servo motor system. A key source of unwanted resonances is the actuator arm. Accordingly, the bandwidths of most servo motor systems are designed such that resonances of the actuator arm occur outside the bandwidth. Each actuator arm has a unique resonance characteristic.
Almost all disc drive systems have at least two actuator arms. In many single disc drive systems, the two outer actuator arms are not accompanied by any inner actuator arms. In many disc drive systems, an outer upper actuator arm and an outer lower actuator arm are accompanied by at least one pair of inner actuator arms. Because most inner actuator arms are installed in a back-to-back configuration, their resonance characteristics are static and fairly equivalent to each other. The outer actuator arms, however, are not installed in a back-to-back configuration and are thus less stable in smaller, thinner actuator arms; the outer actuator arms are free to deflect upon an application of force. As a result, the resonance characteristics, gram load, and pitch of the head gimbal assemblies of the outer actuator arms are different from those of the head gimbal assemblies of the inner actuator arms. Not only is the shape of these resonances distinct but they occur in different locations along the frequency spectrum. Note that in imbedded servo motor systems, the resonance characteristics of each of the inner actuator arms are equivalent and the resonance characteristics of each of the outer actuator arms are equivalent.
If the two groups of actuator arms, the outer upper and lower arms and the inner back-to-back configured arms, resonate at different points along the frequency spectrum, the servo motor system will perform in a less than optimal fashion. These resonances detrimentally affect stability (in the form of shock resistance and feedback vibrations transmitted to the servo motor system) and, to a lesser degree, access times (time required to position the read/write head above the target track). Accordingly, performance of the disc drive system will deteriorate and stability will decrease. The servo motor system cannot accurately position the read/write head over the target track and additional rotation of the motor is needed to correct alignment errors.
Tilt of the flexure arm about the mounting support also affects certain resonances. Tilt is the angular deflection of a flexure about the horizontal plane (vertical deflection). Tilt is not the deflection of the flexure along the horizontal plane.
Prior art has not attempted to equalize the gram loads of the head gimbal assembly of the outer actuator arms and the gram loads of the head gimbal assembly of the inner actuator arms. Since prior art actuator arms were thicker and possessed greater mass, different gram loads on the head gimbal assembly, and thus different resonance characteristics, were not a major problem. However, with lower profile disc drives utilizing thinner and lightweight actuator arms, different gram loads on the head gimbal assemblies have emerged as a significant problem.
Prior art has attempted to minimize the effects of the outer and inner actuator arm resonances by increasing the mass or stiffness of the actuator arm or designing new SEEK methods of positioning the read/write head over the target track. These prior art did not attempt to use the existing actuator arm or a lightweight assembly and adjust the z-height to minimize the negative consequences of distinct arm resonances.
Increasing the mass or stiffness of the actuator arm attenuates, to some degree, the resonances of the arm. However, these heavier arms require higher power requirements to drive the motor. From a manufacturing standpoint, higher mass assemblies reduce manufacturing yields and increase part cost. The use of various part dimensions (e.g., different sizes of the slider for upper and lower actuator arms) also did not address directly the alignment of the resonances to increase performance and stability.
Higher mass assemblies also imply less stability in the form of head lift-off. Head lift-off, or vertical shock resistance, is a function of gram load and magnitude of vertical shock impulses. When a large shock impulse in the vertical direction is applied to the actuator arm, the head gimbal assembly "lifts off" and lands back on the disc surface. This damages the memory disc. Damage potential is lower for higher gram loads. By reducing the gram load, the damage threshold is decreased for equivalent shock impulses and thus, the stability of the disc drive system decreases.
New SEEK head positioning methods do not address resonances or stability. Although low access times may be achieved, the performance of the servo motor is still degraded due to unwanted and uncompensated resonances of the outer actuator arms.